Water in Wood?
Wood when felled will have up to 60% water contained within its cells.
Seasoned wood requires wood to be allowed to dry naturally. This can take several months / years depending on the type of wood, where it is stored and the weather conditions. If stored / stacked in poor conditions for example left in a pile the wood will not season and will eventually begin to rot. The most vulnerable part of the wood is the outer layer called the ‘sapwood’ this will deteriorate faster than the rest of the wood.
Getting rid of the moisture can take several months and in certain species such as Oak up to 2 years this is due to the nature of different species of wood and differing cell structure and density in broadleaf and conifers which will effect the speed of evaporation.
It is clear from the outset to achieve less than 20% moisture needs a certain degree of dedication suitable space and an understanding of how the process works. For example leaving trees in pole length in a heap will not season the wood at all. Evaporation of the water can only be achieved through the cut ends of the logs, a 3 metre length of wood will take years to season particularly the denser species such as Oak and Beech and deterioration of the sapwood will occur making the logs hard to handle and dirty to take in doors. Log stacks will attract a significant amount of beetle activity and fungal decay which in turn will reduce the viability / energy content of the log over time.
Kiln drying wood takes green wood at 50 – 60% moisture content and evaporates the moisture from the processed logs within 10 days. This can only be achieved if the species of the timber is the same and where possible the logs are cut and split to an even size and length. The process for kiln drying logs although less sophisticated is the same as kiln drying sawn timber and the species and the moisture content of the firewood on entering the kiln will dictate the temperature and length of time in the kiln.
The most important point about moisture in firewood is that a log can only be described as being less than 20% moisture content if the centre of the log is less than 20%. To test this it is necessary to take a log and split it in half and test the internal moisture with a moisture meter. Measuring the end or the outside of the log will not give an accurate reading
A reasonable moisture meter can be purchased at B and Q – it is more expensive than some but it is reasonably accurate and has an easy to read scale.
Taking moisture content at the end of the logs as the picture below shows will not measure the moisture content accurately the log must be split and the centre measured.
The log must be split and the moisture at the centre of the split surface measured to get an accurate reading.
Certain species of timber such as Oak can be extremely difficult to dry and the centre of the logs can retain very high levels of moisture.